A high-speed optical fiber communication receiver, in one known form, receives a broadband optical signal and converts it to an electric signal. A transimpedance amplifier is used as the front stage of a typical such optical fiber communication receiver. Characteristics of the transimpedance amplifier can affect the overall system performance.
Referring to FIG. 1, a schematic diagram illustrates a conventional transimpedance amplifier 10 for use in an optical fiber communication receiver. The transimpedance amplifier 10 is operable to convert an input current to an output voltage. The transimpedance amplifier 10 includes a high-speed operational amplifier U having plus (+) and minus (xe2x88x92) inputs and differential outputs labeled Voxe2x88x92 and Vo+. A photo detector PD is connected between a bias voltage source Vb and the minus (xe2x88x92) input of the operational amplifier U. The plus (+) input of the operational amplifier U is connected to ground. The negative output Voxe2x88x92 comprises a voltage output to a post processing amplifier, which comprises a common voltage amplifier. A feedback resistor Rf is connected between the minus output Voxe2x88x92 and the minus (xe2x88x92) input. A peaking capacitor Cp is connected across the operational amplifier U between the plus output Vo+ and the minus (xe2x88x92) input.
Without the peaking capacitor Cp, the bandwidth of the transimpedance amplifier circuit 10 is defined by
BW=1/(2*xcfx80*Rf*Cin*A)
where Cin is the input capacitance including the amplifier input capacitance and parasitic capacitance of the photo detector PD. A is the open loop gain of the operational amplifier U. xcfx80 is the constant 3.1416. The peaking capacitor Cp can balance the input capacitance to widen the bandwidth without sacrifice of the transimpedance gain. For proper operation, it is important to achieve high bandwidth, high gain and low noise.
Since the peaking capacitor Cp forms a positive feedback loop, it has an optimal capacitance value which strongly depends on Cin. If the real peaking capacitance is less than the optimal value, then the transimpedance amplifier 10 can""t reach the expected performance level. If the peaking capacitance is more than the optimal value, then stability of the transimpedance amplifier 10 degrades and overshoot or oscillation may occur.
In a typical application, the operational amplifier U, feedback resistor Rf and peaking capacitor Cp are integrated in a dye with the rest of the system, while the photo detector PD is a separate dye, with both dyes being placed in a single package. The parasitic capacitance of the photo detector PD is difficult to control. Photo detector manufacturers specify the maximum parasitic capacitance and the actual parasitic capacitance varies over quite a large range. In this manner, the peaking capacitor Cp has a fixed value while Cin varies. The system yield rate of optical fiber communication receivers using the transimpedance amplifier 10 shown in FIG. 1 is not stable.
The present invention is directed to overcoming one or more of the problems discussed above, in a novel and simple manner.
In accordance with the invention, there is provided an optical receiver including a transimpedance amplifier and an adjustable peaking capacitor coupled to the amplifier.
Broadly, there is disclosed herein a transimpedance amplifier comprising an operational amplifier having a current input and developing a voltage output. A variable peaking capacitor is connected across the operational amplifier. A control circuit is operatively coupled to the variable capacitor for controlling capacitance to widen bandwidth of the transimpedance amplifier and reach the optimal peaking capacitance at a real operation environment.
It is a feature of the invention that the operational amplifier has differential outputs and the variable peaking capacitor is connected to the operational amplifier to provide positive feedback.
It is another feature of the invention that the control circuit comprises an interface circuit for receiving an external command representing a desired value of the variable peaking capacitor.
It is a further feature of the invention that the interface circuit receives a data signal representing a desired value of the variable peaking capacitor and further comprising a digital to analog converter connected between the interface circuit and the variable capacitor.
It is still another feature of the invention to provide a measurement circuit selectively connected to the voltage output and the interface circuit for measuring voltage output for determining the desired value of the variable peaking capacitor. In one embodiment, the measurement circuit measures signal amplitude. In another embodiment, the measurement circuit measures pulse rise time. In a still further embodiment, the measurement circuit measures overshoot.
It is still another feature of the invention to provide a photo detector connected to the operational amplifier to provide the current controlled input.
It is another feature of the invention that the variable capacitor comprises a varactor, such as a varactor diode or any voltage control capacitors which can be integrated in an integrated circuit.
There is disclosed in accordance with another aspect of the invention, a high-speed optical receiver including a transimpedance front stage amplifier and a post amplifier. The front stage amplifier includes a photo detector connected to a current controlled input of an operational amplifier developing a voltage output. A variable peaking capacitor is connected across the operational amplifier. A control circuit is operatively coupled to the variable peaking capacitor for controlling capacitance to widen bandwidth of the transimpedance amplifier and reach the optimal peaking capacitance at a real operation environment. The post amplifier is connected to the voltage output.
Further features and advantages of the invention will be readily apparent from the specification and from the drawings.